I was thinking how this string of thought could connect to our daily lives. As a family with a toddler, if we fill up too much of our schedule/time in a day, a perturbation to the schedule can break everything. If instead we fill up 63-70% of the schedule and build in Flex Time, we're good!
This stems from the optimal load of self-balancing trees.
A little bit of slack is always useful to deal with the unforeseen.
And even a lot of slack is useful (though not always as it is costly) as it enables to do things that a dedicated resource cannot do.
On the other hand, no slack at all (so running at above 70%) makes a system inflexible and unresilient.
I would argue for this in any circumstance, be it military, be it public transit, be it funding, be it allocation of resources for a particular task.